The overall goal of this project is to understand molecular mechanisms of how the innate immune system senses and responds to bacterial pathogens and the evasion strategies utilized by bacterial pathogens to avoid immune detection. In particular we use Yersinia pseudotuberculosis, which expresses many of the same virulence factors as its close relative and plague pathogen Yersinia pestis, as a model to understand the host- pathogen interface and discover shared features of host innate immune responses to Gram-negative bacterial pathogens. The rapid expansion of multi-drug resistance among Gram-negative bacteria has increased the importance of developing new approaches to antimicrobial therapeutics, and understanding how the innate immune system initially detects bacterial pathogens and the corresponding pathogen evasion strategies holds promise for identifying novel antimicrobials. The Type III secretion system (T3SS) is a broadly conserved virulence determinant that is essential for virulence of many Gram-negative bacterial pathogens from E. coli to Y. pestis, and injects virulence factors into host cells that disrupt cellular signling pathways. However, T3SS activities can also be sensed by cytosolic pattern recognition receptors of the Nod-like receptor (NLR) family and induce host immune responses. Previous studies demonstrated that the Y. pseudotuberculosis and Y. pestis T3SS induces activation of an NLRP3-dependent inflammasome, resulting in cell death and caspase-1 dependent cytokine secretion. Furthermore, studies demonstrated that the essential Yersinia virulence factor YopK, which modulates the pore-forming activity of the T3SS and limits translocation of other Yop effector proteins, prevents inflammasome activation. Inflammasome activation plays an important role in host defense against Yersinia, as the virulence defect of YopK-deficient bacteria is restored in mice that lack inflammasome components. However, the molecular basis for how cells sense the activity of Yersinia's T3SS and how YopK prevents this sensing is not known. We propose three Specific Aims to address this important gap in our knowledge. First we will test the hypothesis that delivery of translocon components into the host cell cytosol activates the NLRP3 inflammasome through disruption of intracellular compartments. Second we will test the hypothesis that YopK prevents NLRP3 inflammasome activation by binding to the translocon and limiting injection of translocon components into the cell. Third, we will test te hypothesis that inflammasome activation in vivo controls Yersinia infection via production of caspase-1 dependent cytokines that induce activation of specific immune cell subsets that promote bacterial clearance.